1. Field of the Invention
The present invention relates to a dome shaped spring and a switch including a dome shaped spring.
2. Description of Related Art
Conventionally, an operation section of an electronic device such as a cellular phone, etc. is known to have a configuration using a push button switch with a clicking action. The push button switch with a clicking action can provide an operator a clicking feel when the pushing is input. The push button switch with a clicking action includes a dome shaped contact spring.
As the above described push button switch, there is known a switch including a dome shaped contact spring where an entire surface is a spherical surface. With reference to FIG. 14, an apparatus configuration of a switch 100 including a conventional dome shaped contact spring where an entire surface is a spherical surface is described. FIG. 14 shows a cross sectional view of the configuration of the switch 100 including a dome shaped contact spring 21.
As shown in FIG. 14, the switch 100 includes a dome shaped contact spring 21, substrate 3, fixed contacts 4, 5 and 6. The dome shaped contact spring 21 is a spring for a contact composed of a metal conductor in a dome shape where the entire surface is a spherical surface. FIG. 14 is a cross sectional view of a face passing a movable contact 21a in a position in a center of a plane of the dome shaped contact spring 21.
The substrate 3 is a substrate on which the dome shaped contact spring 21 is placed. The substrate 3 is provided with fixed contacts 4, 5 and 6. The fixed contacts 4, 5 and 6 are electric contacts including a metal conductor. The fixed contacts 4 and 5 are constantly in contact with an edge section of the dome shaped contact spring 21. The fixed contact 6 is provided in a position corresponding to the movable contact 21a of the dome shaped contact spring 21.
The operation of switch 100 is described with reference to FIG. 15. FIG. 15 shows a characteristic of an operation load F to a displacement of the movable contact 21a in switch 100.
With the switch 100, the dome shaped contact spring 21 is pushed down from above by the operator. When the movable contact 21a of the dome shaped contact spring 21 is in contact with the fixed contact 6, the fixed contacts 4 and 5 are in an electrically conductive state with the fixed contact 6 through the dome shaped contact spring 21. With this conductive state, an electric signal showing the dome shaped contact spring 21 is pushed down is output from the operation section including the switch 100.
With such switch 100, it is assumed that the operator pushes the movable contact 21a of the dome shaped contact spring 21 from the above with an operation load F [gf]. In an initial position where force is not applied to the movable contact 21a, the operation load F and a displacement [mm] of the movable contact 21a in a vertical direction with respect to the substrate 3 is set to 0.
From the initial state of the switch 100, the operator pushes the switch 100 to initiate the increase of the operation load F. As shown in FIG. 15, the operation load F increases almost in proportion from displacement 0 to displacement S11. Then, buckling occurs in the dome shaped contact spring 21 at operation load F11 corresponding to displacement S11. Then, the center portion of the dome shaped contact spring 21 is inverted and the movable contact 21a is displaced with an operation load smaller than the operation load F11. Next, the operation load continues to decrease until the movable contact 21a reaches the displacement S12 to be in contact with the fixed contact 6. Then, when the operation load F12 corresponding to the displacement 12 is gone, the dome shaped contact spring 21 is inverted in the opposite direction, and returns to the initial state.
Also, there is known a switch including a dome shaped contact spring including a spherical section with a spherical surface and a base portion (for example, Japanese Patent No. 4079431, Japanese Patent No. 3753676, Japanese Patent Application Laid-Open Publication No. 2004-139997, and Japanese Patent Application Laid-Open Publication No. 2000-322974). A switch 200 including a dome shaped contact spring including a spherical section and a base section is described with reference to FIG. 16. FIG. 16 shows a cross sectional view of a configuration of the switch 200 including the dome shaped contact spring 22.
As shown in FIG. 16, the switch 200 includes a dome shaped contact spring 22, substrate 3, and fixed contacts 4, 5 and 6. The dome shaped contact spring 22 includes a spherical section 22A and base section 22B. The spherical section 22A is a spring for the contact including a dome shaped metal conductor with a spherical surface. The base section 22B is a circular cone shaped metal conductor connected to the spherical section 22A. The spherical section 22A includes a movable contact 22a which is a portion movable with an operation load. The base section 22B is a portion which hardly moves with the operation load.
In the switch 200, compared to the switch 100, the dome shaped contact spring 22 enables the height from the substrate 3 to be lower. Therefore, the switch 200 can be made smaller. Residual stress for clicking action is applied to the dome shaped contact spring 22 with press work in manufacturing.
Lately, there is a demand for electronic devices such as cellular phones, etc. including the switch to be made even smaller. However, with the conventional switch 200, when the height from the substrate 3 is made lower, the buckling does not occur even when the operation load is applied and the clicking action cannot be obtained.